route/route6-change-gw: Fix error "RTNETLINK answers: File exists"

Use ip route change instead of adding and deleting route (the same
approach route4-change-gw uses).
This prevents occasional rtnetlink error "RTNETLINK answers: File exists"
when route already exists.

Signed-off-by:jian yang <[email protected]>
[pvorel: adjusted commit message]
Signed-off-by: Petr Vorel <[email protected]>

Author: fun2yang

testcases/network/stress/route/route6-change-gw

@@ -245,8 +245,7 @@ test_body()
 	    route -A inet6 del ${dst_network}/64 gw ${IPV6_NETWORK}:${pre_rhost_part_hex} dev $lhost_ifname
 	    ;;
 	    2)
-	    ip -f inet6 route add ${dst_network}/64 via ${IPV6_NETWORK}:${rhost_part_hex} dev $lhost_ifname
-	    ip -f inet6 route del ${dst_network}/64 via ${IPV6_NETWORK}:${pre_rhost_part_hex} dev $lhost_ifname
+	    ip -f inet6 route change ${dst_network}/64 via ${IPV6_NETWORK}:${rhost_part_hex} dev $lhost_ifname
 	    ;;
 	esac
 	if [ $? -ne 0 ]; then